This invention relates generally to infrared photodetectors and, more particularly, to superconducting infrared photodetectors having thin granular film detector elements.
Infrared photodetectors are widely used in surveillance, monitoring, and imaging systems and are of two general types. Bolometric or thermal photodetectors rely on detector elements that undergo a change in some temperature-dependent parameter, such as resistance, when uniformly heated by infrared radiation. Bolometric photodetectors are typically broadband, but tend to have either a slow response time or poor sensitivity. Quantum or nonequilibrium photodetectors do not rely on a uniform heating of the detector elements and, therefore, usually provide both a fast response time and good sensitivity.
Quantum-type detector elements are frequently fabricated from either semiconducting or superconducting materials. Semiconducting materials generally provide good quantum detection of photons at energy levels corresponding to the energy gaps of these materials. The energy gaps of semiconducting materials are on the order of 1 eV, which is in the near infrared portion of the electromagnetic spectrum. Superconducting materials generally provide good quantum detection of photons at much lower energy levels because of the much smaller energy gaps of these materials. The energy gaps of low-temperature superconducting materials are on the order of 1 meV, which is in the millimeter wave portion of the spectrum.
Thin superconducting granular films, however, have recently shown considerable promise in detecting radiation over a wide range of the electromagnetic spectrum, including the desirable infrared portion of the spectrum. These granular films contain small grains of superconducting material which form a randomly connected array of weakly coupled superconductors. The weakly coupled superconductors promote the formation of oppositely-polarized fluxons, which are driven toward opposite sides of the film when subjected to a bias current. Incident infrared radiation causes an increase in this fluxon flow, generating a measurable voltage change. Unfortunately, infrared photodetectors that utilize these granular films have typically suffered from poor sensitivity and low signal-to-noise ratios, and have been difficult to implement in focal plane arrays. Accordingly, there has been a need for a superconducting infrared photodetector having thin granular film detector elements that does not suffer from these limitations. The present invention is directed to this end.